Smartphones typically include GPS (Global Positioning System) circuitry. The GPS circuitry of modern smartphones may be sufficiently sensitive to determine the smartphone's location inside buildings. Unfortunately the accuracy of GPS inside buildings is poor. This is because GPS positioning is based on travel time differences from a number of satellites, and when all the satellites are received through the same window, the travel time differences are the same regardless of the smartphone's position within the building.
In order to track the location of persons in places where GPS positioning is insufficient, it is customary to use positioning devices that is based on signals whose one or more parameters depend from the position of the device in a known or predictable manner. For instance, such positioning may be based on radio signal strength or quality observations. In some implementations a positioning device observes signal parameters, such as strength or quality, from various radio beacons, such as WLAN (wireless local-area network) access points or dedicated positioning transmitters. These observations are then used, either in the device itself or in an external positioning engine, to determine the device's position. In other implementations the scheme is reversed and the fixed stations observe a signal transmitted by the device to be positioned. Alternatively or additionally, signal content may be used. For instance, detection of a specific transmitter's carrier signal is in itself a useful indicator of position, regardless of the strength or quality of that signal. Detection of a transmitter's identity may be sufficient for positioning the device in cases wherein the transmitter's signal cannot propagate through walls. Visible light, infrared radiation, microwave radiation and ultrasound are examples of signals that are practically confined to the spaces in which they are transmitted, and serve as indicators of the device's presence in the respective spaces.
It is customary to use the term “tag” or “positioning tag” to mean a positioning device whose sole or primary purpose is to keep track of a person or object the tag is attached to. Such positioning techniques and tags are well known in the art and can be obtained from a number of sources, including the assignee of the invention described later in this document.
A residual problem in existing positioning tags relates to generation of an alert signal in case of distress or emergency. For instance, commonly-owned PCT application WO2009/122000 describes, in connection with FIG. 9, a positioning tag with a push button for triggering an alert condition. In the prior art systems, the wearer of the positioning tag needs time to find the alert button. If the person is using protective gloves or mittens, it may be impossible to press a small button.